To drill in the shallow waters (less than 100 feet depth) of the outer continental shelf off Alaska year-round, new types of exploratory drilling structures are needed. This is mainly because of the enormous environmental loads imposed on drilling structures by sea ice conditions during the winter months and during summer break-up. The ice loadings will generally exceed normal offshore structure design loadings, such as storm wave conditions. As a consequence, structures designed to operate under these conditions will tend to be more massive and structurally stronger than conventional offshore exploratory structures.
In addition to remaining structurally intact during operations in sea ice conditions, the drilling structure must remain stationary, on site, during these same conditions. Thus, the structure must have little or no horizontal movement when subjected to ice loadings. To accomplish this, it must transfer the substantial horizontal ice loads to some sort of foundation, typically the sea floor. For shallow water depths, this can usually be accomplished by load transfer between the structure and the foundation by two main means. The first involves frictional load transfer between the structure and foundation. It relies on the horizontal shear resistance to keep the structure sited. The second involves mechanical means of load transfer through the use of piles driven into the sea floor. The piles absorb the load from the structure and then transfer it into the sea floor. A third possible variation involves the use of both friction and piles to effect load transfer.
Several of the above factors combine to complicate oil exploration in arctic regions where many of the most promising sites lie under relatively shallow water. The same shallow water sites are also invaded by drifting ice features that will apply very large lateral loads to a stationary sea surface-protruding structure.
It is also possible that these loads could be applied to the structure at any time of the year. It is therefore desirable to have a structure which can be installed and brought up to full strength quickly. Piles could be used to provide a significant portion of the total strength requirement but they take a relatively long time to install, so there is a danger that a pile-retained structure may be subjected to an ice load before it is completely "up to strength".
As an alternative, the structure could be ballasted with a high density material such as iron ore or gravel, but iron ore is not locally available at all and gravel is relatively scarce in many arctic regions. Either one of these materials would have to be transported in large quantities over distances greater than 100 miles. This becomes prohibitively expensive in an arctic environment. They also require substantial amounts of time and effort to put in place or remove to refloat the structure.
In general, an offshore structure for drilling exploratory wells in shallow ice-covered waters, such as arctic waters, desirably meets many, if not all, of the following criteria:
1. The structure should have a draft as shallow as possible in the lightship condition. This would allow the structure to operate in water depths as shallow as possible, thus maximizing its operational versatility.
2. The structure should utilize an abundant, cheap, readily available ballast material. This would minimize problems relating to the use and acquisition of often scarce, expensive ballast materials.
3. The structure should have the ability to move onto site, take on ballast and be ready for drilling operations to begin, and resist environmental ice loadings, as soon as possible. Similarly, the structure should be able to de-ballast, pick-up and be able to move offsite in a time period as short as possible. This would allow for a move to a different site during the short summer season in the target region.
4. The structure should have the ability to conduct normal drilling operations year-round. Due to large periods of complete ice cover in target areas, this would necessitate that the structure be able to store sufficient quantities of drilling material and supplies to allow continuous operation for extended periods of time, as up to 270 days, without major resupply of the structure.
5. Based on the above requirement for large amounts of drilling consumables, the structure should have a large storage capacity for such materials. Since several of these items are quite bulky and heavy, the structure must provide sufficient space and structural load-carrying capacity to store these materials.
6. The structure should minimize any necessary site preparation and alteration. This will reduce the extent of expensive, slow offshore foundation work and cause the least possible adverse impact to the existing environment. By reducing foundation preparation requirements, the use of expensive, scarce materials such as gravel and sand typically used for this purpose is minimized.